The name and description don't have to be Product Identity; the stat block (including special powers), however, must be Open Game Content.
Only the name and description. The stats are Open Game Content regardless of your say-so, because they were OGC from the beginning.
Technically, yes, and that
was the common understanding for a long time. However, no one, as far as I recall, actually tried pushing that and just abided by the OGC declarations even if they were more restricted than should have been allowed to be. Especially add in the complexities of how exactly the word "derived" would be interpreted legally.
(If I write a feat, does simply being a feat make it derived? Or just the parts of the feat that are more specifically derived like "You can add your Charisma modifier to your melee weapon attack rolls" but other text is not? What if I call it a Shmeat that it not a feat, but you can use it in place of a feat? Etc. Etc. It's unfortunately too indistinct for anyone willing to take the risk.)
Fast forward to now where lack of enforcement and rampant misunderstanding of the OGL, and that's sadly a lost cause. Trying to claim someone else's content as automatic OGC would likely be a PR nightmare, unfortunately, even if it technically did actually make it to court, the license might technically stipulate that as you say.
So, in practice, rather than in theory, nothing has been forced to be OGC due to being derived. It should have been, but even back in the 3.0 heyday, most publishers would (and did!) say "That is OGC whether they like it or not, but it's not worth the risk of challenging their awful OGC declaration."
Therefore any claims of "derived content is automatically OGC" are basically academic and of no practical relevance.
